EXTERNAL.CHARACTERS AND HABITS S9
their native haunts describe them as active, agile creatures. They run about on the ground in a quadrupedal manner, using their fore and hind limbs almost equally. They also jump off one foot which is used like a spring-board. In the trees they swing about as actively as some Gibbons. When the animal has been in captivity for a long time it develops stiffness of the joints, and its active movements are replaced by an awkward gait in which the body is dragged along, the forearms being used like crutches. It must be emphasized, therefore, that the mode of locomotion of an old menagerie specimen is quite unnatural.
Occasionally the Chimpanzee stands upright, and it- is clearly seen at these times that the narrow sole is planted firmly on the ground, there being no plantar arch. It only remains erect for a very short period, for the mechanisms which allow Man to remain upright and be bipedal are poor or absent. Man's upright position is due to the following factors acting in harmony :—
(1) The centre of gravity is low.
(2) The ilio-femoral ligament is a strong band.
(3) The structure of the knee-joint with its ligaments does not permit the thigh and leg to be rigidly locked.
(4) The structure of the ankle-joint.
(5) The muscular system is more divided and the muscles can co-operate to suit awkward attitudes.
(6) The regulating powers of the central nervous system have reached a high degree of perfection.
These factors will receive detailed attention in subsequent chapters.
The arms are used as hook-like suspenders, for the diminutive thumb cannot convert the hand into a